1. Field of the Invention
The present invention relates to an interferential position measuring arrangement for determining the relative positions of two objects which are movable with respect to each other at least in the measuring direction, and which includes a light source, an optical element arranged downstream of the light source, a scale grating arranged downstream of the light source, respective scanning gratings, a detection arrangement, and optoelectronic detector elements arranged in at least three spatial directions.
2. Discussion of Related Art
Interferential position measuring arrangements for a precise position determination, which utilize diffraction effects at gratings for generating high-resolution, position-dependent scanning signals, are known. Phase shifts which are proportional to the path change result in the partial beams with deflected orders of diffraction in the course of a movement of a scale grating in relation to a scanning unit. For evaluating, or detecting, the respective phase shifts, the various split partial light beams, or orders of diffractions, are superimposed and caused to interfere. In the case of a movement a periodic modulation of the interference strip pattern results, which is detected by a suitable optoelectronic detector arrangement. In connection with such interferential position measuring arrangements reference is made, for example, to Chapter 4, pages 47 to 52 of a dissertation of J. Willhelm, entitled “Dreigitterschrittgeber” [Three-grating Incremental Generator], 1978.
A further variation of interferential position measuring arrangements is known from FIG. 13 in U.S. Pat. No. 6,005,667, the entire contents of which are incorporated herein by reference. Following the formation of a beam by a lens, a beam of rays emitted by a light source impinges on a scale grating arranged in the area of the beam waist. The impinging beam of rays is split by the scale grating into +/− orders of diffraction, or into corresponding partial beams of rays, which spread out away from the optical axis. Thereafter, the partial beams of rays reach further diffracting scanning gratings, which deflect the impinging partial beams of rays back in the direction of the optical axis. A detection arrangement, which has four phase-shifted partial areas for generating four scanning signals, each phase-shifted by 90°, is arranged in a detection plane in which the partial beams of rays are brought to interference.
In addition, reference is made here to U.S. Pat. No. 5,574,558, the entire contents of which are incorporated herein by reference, which discloses a further interferential position measuring arrangement.
In connection with such high-resolution position measuring arrangements, the demands made on the mounting tolerances have been shown to be particularly critical. This means that as a rule the mounting tolerances are very narrow, which causes a large outlay for assembly and adjustment in turn. Added to this are demands for an arrangement of the smallest possible dimensions, in particular a scanning unit of the smallest possible dimensions, and the greatest immunity to soiling or contamination. Basically, in connection with such position measuring arrangements the best possible quality of the resulting scanning signals is important, especially if further electronic processing, for example an interpolation of the signals, is provided. However, none of the interferential position measuring devices in the publications cited meets all mentioned requirements in a satisfactory way.
Thus, the arrangement known from U.S. Pat. No. 6,005,667 continues to be susceptible to soiling of the scale grating, in spite of a defined immunity regarding misalignments of the scale grating and the scanning unit.
It should be mentioned that the known device in U.S. Pat. No. 5,574,558 is very sensitive to fluctuations of the distance between the two gratings which can be moved in relation to each other, i.e. only small mounting tolerances exist in this direction. This is caused by the divergent, or non-collimated illuminating beam of rays impinging on the first grating.